A theoretical investigation into the luminescent properties of d⁸-transition-metal complexes with tetradentate Schiff base ligands

A theoretical investigation on the luminescence efficiency of a series of d8transition-metal Schiff base complexes was undertaken. The aim was to understand the different photophysics of [M-salen]ncomplexes (salen=N,N'-bis(salicylidene)ethylenediamine; M=Pt, Pd (n=0); Au (n=+1)) in acetonitrile solutions at room temperature: [Pt-salen] is phosphorescent and [Au-salen]+is fluorescent, but [Pd-salen] is nonemissive. Based on the calculation results, it was proposed that incorporation of electron-withdrawing groups at the 4-position of the Schiff base ligand should widen the3MLCT-3MC gap (MLCT=metal-to-ligand charge transfer and MC=metal centered, that is, the dd excited state); thus permitting phosphorescence of the corresponding PdIISchiff base complex. Although it is experimentally proven that [Pd-salph-4E] (salph=N,N'- bis(salicylidene)-1,2-phenylenediamine; 4E means an electron-withdrawing substituent at the 4-position of the salicylidene) displays triplet emission, its quantum yield is low at room temperature. The corresponding PtIISchiff base complex, [Pt-salph-4E], is also much less emissive than the unsubstituted analogue, [Pt-salph]. Thus, a detailed theoretical analysis of how the substituent and central metal affected the photophysics of [M-salph-X] (X is a substituent on the salph ligand, M=Pt or Pd) was performed. Temperature effects were also investigated. The simple energy gap law underestimated the nonradiative decay rates and was insufficient to account for the temperature dependence of the nonradiative decay rates of the complexes studied herein. On the other hand, the present analysis demonstrates that inclusions of low-frequency modes and the associated frequency shifts are decisive in providing better quantitative estimates of the nonradiative decay rates and the experimentally observed temperature effects. Moreover, spin-orbit coupling, which is often considered only in the context of radiative decay rate, has a significant role in determining the nonradiative rate as well. Low, but not ignored: Low-frequency normal modes are usually not considered to be important in determining nonradiative decay rates because they are not effective energy acceptors. The importance of these low-frequency normal modes in estimating knrby using a convolution approach is highlighted (see figure). KGaA, Weinheim.